1. Field of the Invention
The present invention relates to semiconductor laser elements of a strained quantum well type and a method for the production of the semiconductor laser elements.
2. Description of Prior Art
A semiconductor laser element formed on a GaAs substrate with an active layer of a strained quantum well construction provided with an In.sub.x Ga.sub.1-x As (x=0.0 to 0.5) strained quantum well layer and a GaAs barrier layer is expected as a light source of wavelength of 0.9 to 1.1 .mu.m which just comprises of a raving in a conventional lattice matched type laser such as GaAs/AlGaAs and InAsP/InP.
In case of semiconductor laser elements, as a clad for confining carrier and light in an active layer, a semiconductor should be used which has a permeability with respect to light having an oscillation waveform, is smaller in refractive index than that of the active layer (or a layer for confining light near the active layer), and is large in energy gap.
In the conventional semiconductor laser element of an In.sub.x Ga.sub.1-x As strained quantum well type, Al.sub.w Ga.sub.1-w As of w&gt;0.2 is used as a clad.
FIG. 8 shows a conventional semiconductor laser element of an In.sub.x Ga.sub.1-x As strained quantum well type.
In FIG. 8, an n-type GaAs substrate 1 having approximately 350 .mu.m of thickness is formed thereon with an n-type GaAs buffer layer 2 having approximately 0.5 .mu.m of thickness and an n-type Al.sub.0.3 Ga.sub.0.7 As clad layer 3 having approximately 1.5 .mu.m of thickness through epitaxial growth means such as MBE method or MOCVD method.
Further, in FIG. 8, the n-type GaAs substrate 1 is formed at its predetermined position with an essential portion 4 including an active layer provided with an In.sub.0.35 Ga0.65As strained quantum well layer, a GaAs barrier layer, etc., and a light confining layer, a p-type Al.sub.0.3 Ga0.7As clad layer 5 having 1.5 .mu.m of thickness and a p-type contact layer 6 having 0.2 .mu.m of thickness.
The details of the essential part 4 is clearly shown in FIG. 9.
In FIG. 9, two upper and lower GaAs light confining layers 7 has 1500 .ANG. of thickness, a GaAs barrier layer 8 between these light confining layers 7 has 100 .ANG. of thickness and an In.sub.0.35 Ga.sub.0.65 strained quantum well layer 9 has a thickness of 40 .ANG..
A double hetero construction formed on the GaAs substrate 1 is formed with a current restricting layer and an electrode and is applied with microworking such as element separation to prepare a laser chip.
One example of prior art has been described above. The semiconductor laser element has various modes such as a ratio of composition of mixed crystal, the number of layers of strained quantum wells, thickness of the layers, etc. As the light confining layer, there is well known a GRINSHC construction using AlGaAs in which the ratio of Al composition is parabolic.
Next, the process for working the bridge waveguide path into a strained quantum well type semiconductor laser element will be described hereinafter with reference to FIGS. 10(a) to 10(d).
In the process shown in FIG. 10(a), a resist 10 is patterned on a p-type GaAs contact layer 6 through means such as photolithgraphy.
In the process shown in FIG. 10(b), the resist 10 is used as a mask. The GaAs contact layer 6 of a double hetero construction and the upper clad layer 5 are subjected to etching till the depth of the etching reaches about 0.2 m of the active layer 4.
As etching liquids in etching AlGaAs/GaAs type, there can be used a mixed solution of sulfuric acid and hydrogen peroxide, a mixed solution of tartaric acid and hydrogen peroxide, a mixed solution of ammonia and hydrogen peroxide or dry etching such as chlorine (for example, reactive ion beam etching).
In the process of FIG. 10(c), means such as spattering is used to form a surface of an epitaxial film with an etching mask 11 in the form of a film such as SiO.sub.2, SiN, etc.
In the process of FIG. 10(d), means such as photolithgraphy is used to form an etching portion 12 in a stripe-like SiO.sub.2 as a patterned resist mask.
In the thereafter processes, electrodes are formed on both upper and lower surfaces of a laminate construction, and microworking such as element separation is applied thereto.
The technical process with regard to the aforementioned semiconductor laser element include oxidation of Al, compressive stress from a substrate lattice and defective etching process, which will be described hereinafter.